One presently known assembly of this kind consists of an annular member which is adapted to be mounted to the light fixture and of a base member which is adapted to be mounted to the supporting structure. The inner engagement between the base member and the annular member is effected through a relative rotational movement such that circumferentially spaced lugs on the inner rim of the annular member frictionally engage corresponding lugs on the outer peripheral wall of the base member. However, this frictional metal-to-metal contact has drawbacks in that a tight rigid engagement is rarely achieved and, in cases where it is achieved, it is rather a jammed engagement where disengagement between the annular and base members may in time become extremely difficult. To maintain an adequate rigid and sealing engagement between the parts (in order to prevent disengagement through vibration when a rigid engagement is not achieved), a pressure screw extending through the base member is used to exert force against the annular member and to lock the two members together. However, tightening this screw causes separation between the two parts in areas where there are no lugs thereby defeating its own purpose of preventing rain or snow from penetrating inside the assembly. It is essential that no gap be present because of the possibility of having rain or snow penetrating inside the assembly to corrode and damage the light socket and/or ballast. Indeed, present systems relying on aluminum-to-aluminum or steel-to-steel contact are difficult to operate after exposure to corrosive atmosphere often encountered in street and area lighting conditions.
Furthermore, it has been found that in cases where the light fixture is not transparent, it is extremely difficult to mount the fixture to the annular member. The latter is a cast member; all parts are therefore integral such as the threaded pins which protrude on the fixture side of the annular member and are used to secure two semispheric brackets which, in turn, serve to tighten the light fixture to the annular member. The user must therefore work in a very ackward position, that is, he must insert his hands through the central opening of the annular member to reach the fastening and the fastened parts where it becomes very difficult to secure the parts together.